Couplers are important devices for many communication applications and in integrated photonic circuits. Couplers are needed to combine or separate signals and to interconnect the various points of a communications network. The are many constraints involved with coupler design, however. Such constraints include the laser structure with which the coupler may be used, the number of ports, sensitivity to light transmission direction, wavelength selectivity, type of fiber, signal attenuation, and cost. High-power distributed feedback (DFB) lasers are light sources of choice in many optical communications systems, which makes coupling the laser light out of planar waveguides and possibly into fibers a crucial technological issue. Traditionally, one-dimensional grating couplers (GCs) and focusing GCs have been used to couple laser light out of a waveguide plane and achieve coherent scattering of the light. See, e.g., A. Katzir et al., APPL. PHYS. LETT. 30, 225 (1977); Loewen et al., DIFFRACTION GRATINGS AND APPLICATIONS (M. Dekker, NY 1997); D. Heitmann et al., APPL. PHYS. LETT. 37, 585 (1980); Hatakoshi et al., APPL. OPT. 23, 1749 (1984); P. Borsboom et al., J. OPT. SOC. AM. A12, 1142 (1995).
There are drawbacks, however, with such one-dimensional gratings, particularly with regard to the directionality of the output light. The direction of the output light naturally affects how well the light may be coupled into receivers or other devices, e.g., planar waveguides and fibers. Both one-dimensional grating couplers and focusing grating couplers have periodicity in a single spatial direction. One-dimensional GCs have straight grooves, whereas focusing GCs, also called grating lenses, have a curvelinear grating. The direction of light output from a coupler is determined by phase-matching the scattered wave to the guided wave. As shown in FIG. 1A, a one-dimensional grating couplers e.g. 1 couple light to a cylindrical wave, necessitating the use of additional optics to direct the light into a fiber. As shown in FIG. 1B, focussing grating couplers e.g. 3 focus light to a point 4 in space in the vicinity of the grating at a distance on the order of the grating size. With focusing couplers, a receiver may only be placed at a certain fixed distance from the coupler, and in the far field, light is coupled to a spherical wave.
Additionally, semiconductor lasers using organic or polymeric materials and electrically-driven laser action have recently attracted a great deal of interest. Organic solid-state lasers have the potential to provide a compact low-cost laser source over a broad range of wavelengths throughout the visible spectrum. Organic lasers also influence research in other areas and have led to advances with both organic and inorganic semiconductor lasers, as described in A. Dodabalapur et al., “Organic Solid-State Lasers: Past and Future,” SCIENCE Vol. 277 (Sep. 19, 1997), at pp. 1787–1788, incorporated herein, and in U.S. patent application Ser. No. 09/385,167, referenced above (hereinafter the “'167 application”).
Examples of advances in organic or inorganic semiconductor lasers include the successful realization of distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers with dye-doped polymers and the widespread use of InP-based DFB and DBR lasers. Such lasers exhibit superior single frequency operation and high-speed modulation characteristics, e.g., as compared with Fabry-Perot lasers. DFB and DBR are deployed in many commercial systems including long-haul fiber optic communication systems. An assembly comprising a DBR or DBF laser monolithically integrated with an off-plane computer-generated waveguide hologram and semiconductor amplifier is disclosed in Feng et al., “Grating-Assisted Surface-Emitting Laser Transmitter with Image-Forming Capability” IEEE Photonics Tech. Letters, Vol. 10, No. 12 (December 1998). Feng et al. define their computer-generated hologram as “essentially a surface relief grating-like” structure the design of which is not clearly defined.
DFB and DBR lasers are examples of one-dimensional photonic-crystal lasers since they possess one-dimensional gratings as part of their structure. One-dimensional photonic crystal lasers provide many advantages. For example, the density of states is sharply peaked at the edges of the air and dielectric bands, leading to low thresholds. Although two-dimensional photonic crystal lasers have been demonstrated {see, e.g., M. Meier et al., APPL. PHYS. LETT. 86, 3502 (1999), which is incorporated herein}, for many applications one-dimensional lasers remain preferred.
As may be appreciated, those involved in the field of communications systems and semiconductor devices continue to seek to develop new designs to improve device efficiency and performance and to allow for the use of new materials, such as GaN and plastics. In particular, it would be advantageous to provide a coupler that avoids the directionality restraints of one-dimensional and focusing GCs that is compatible with one-dimensional photonic crystal lasers such as DFB and DBR lasers.